The present invention relates to a magnetron in use for a high-frequency heating device, such as a microwave oven.
In a conventional magnetron, as shown in FIG. 5, a filter case 1 contains therein a choke coil 5, a cathode input wire 6, and a capacitor 7. The choke coil 5 is formed with a core type inductor 3 and a air-core inductor 4, which are connected in series. The core type inductor 3 has a high-frequency absorbing member 2 of a bar-like ferrite located within a winding thereof. The air-core inductor 4 does not have a high-frequency absorbing member in a winding thereof. The cathode input wire 6 is connected to a first end 5a of the choke coil 5, closer to the air-core inductor 4. The capacitor 7 has a capacitor terminal connected to a second end 5b of the choke coil 5, closer to the core type inductor 3.
In the magnetron, the air-core inductor 4 of the choke coil 5 is inserted between the cathode input wire 6 and the core type inductor 3 of the choke coil 5. This structure succeeds in solving a problem of insulation failure resulting from the burning of an insulating film coated on the winding of the choke coil 5, and another problem of the cracking of the high-frequency absorbing member 2 (see, for example, JP-B-57-017344<Japanese examined patent publication number: SHOU 57-17344>).
However, the conventional magnetron has a capability of attenuating only the noise at 400 MHz or lower even if the number of turns of the winding of the core type inductor 3 is adjusted, and further of attenuating only the noise within a range of 700 to 1000 MHz even if the number of turns of the winding of the air-core inductor 4 is adjusted. Therefore, there is a problem that the noise within a range from 500 to 700 MHz will interfere with communication radio waves.